ci: Move to GCC 5.4 (Xenial default) for prebuilts (#946)
ci: Move to GCC 5.4 (Xenial default) for prebuilts (#946)

This is part 1 of a change to switch us over to a GCC 5.4 so that
we get a thread safe std::string implementation. Will follow up
with another commit with CI and doc changes.

craffert0/envoy,eklitzke/envoy,htuch/envoy,envoyproxy/envoy,lizan/envoy,tdmackey/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,sebrandon1/envoy,lyft/envoy,sebrandon1/envoy,rlazarus/envoy,amalgam8/envoy,rlazarus/envoy,tdmackey/envoy,envoyproxy/envoy,craffert0/envoy,tdmackey/envoy,istio/envoy,eklitzke/envoy,fengli79/envoy,istio/envoy,envoyproxy/envoy,craffert0/envoy,amalgam8/envoy,envoyproxy/envoy,jrajahalme/envoy,lizan/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,eklitzke/envoy,fengli79/envoy,dnoe/envoy,rlazarus/envoy,htuch/envoy,istio/envoy,tschroed/envoy,envoyproxy/envoy-wasm,tschroed/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,istio/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,lizan/envoy,sebrandon1/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,lizan/envoy,rlazarus/envoy,lyft/envoy,tdmackey/envoy,dnoe/envoy,dnoe/envoy,craffert0/envoy,tschroed/envoy,lizan/envoy,amalgam8/envoy,dnoe/envoy,sebrandon1/envoy,envoyproxy/envoy-wasm,lyft/envoy,istio/envoy,envoyproxy/envoy,istio/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy,htuch/envoy,envoyproxy/envoy,amalgam8/envoy,sebrandon1/envoy,istio/envoy,dnoe/envoy,eklitzke/envoy,eklitzke/envoy,fengli79/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lyft/envoy,fengli79/envoy